Fix intra-doc link warnings; add clawsync-fs README and metadata

Doc fixes (20 warnings → 1 benign name-collision warning):
- clawsync-transport/quic.rs: QuicConfig::with_cert doesn't exist → backtick
- clawsync-onion/iblt.rs: bare `insert` link → backtick
- clawsync-core/lib.rs: simd_cdc is feature-gated → backtick
- clawhdf5-onion/annotation.rs: RevisionEntry/BranchEntry are in external
  clawhdf5-format crate, not re-exported → backtick
- clawhdf5-onion/gc.rs: compact_dead_epoch_revisions is private; flush links
  broken → backtick
- clawhdf5-onion/provenance.rs: RevisionEntry from external crate → backtick
- clawhdf5-onion/reader.rs: reconstruct_revision/revision_pages → Self:: prefix
- clawhdf5-onion/writer.rs: REV_FLAG_SNAPSHOT → crate::format:: path;
  reconstruct_revision → Self:: prefix
Remaining warning is `format` module/macro name collision — not a broken link.

clawsync-fs crate metadata:
- Add readme, keywords, categories to Cargo.toml
- Write README.md (CDC protocol diagram, module overview, usage examples)

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

crates/clawsync-fs/Cargo.toml

@@ -6,6 +6,9 @@ edition.workspace    = true
 rust-version.workspace = true
 license.workspace    = true
 repository.workspace = true
+readme       = "README.md"
+keywords     = ["sync", "rsync", "cdc", "delta", "filesystem"]
+categories   = ["filesystem", "network-programming"]
 
 [dependencies]
 clawsync-core      = { workspace = true }